Track circuits are well known and widely used in railway signalling systems. For the purposes of this description, a track circuit may be defined as a circuit which is designed to detect and respond to the presence of a train within the boundaries of a specified section of track. There may be loss of life and/or equipment damage if a track circuit should fail to indicate the presence of a train. In response to the detection of a track, a track circuit may provide signals at a highway crossing for lowering gates and providing other audible and/or visual signals. If a track circuit should indicate the presence of a train when there is no train, considerable inconvenience may be caused. However, such inconvenience is considered more desirable than the failure of the track circuit to indicate the presence of a train when there is a train. Accordingly, part of the philosophy of design of track circuits is that under no circumstances should the track circuit indicate there is no train when there is, in fact, a train. The track circuit should respond, as described under normal conditions and even when there is a wide variety of faults and/or malfunctions such as blown fuses, voltage out of tolerance, some wiring errors, insulation failure, dirt, dust or moisture or defective components, etc. In summary, the track circuit should never indicate the absence of a train when there is a train.
The relay which provides the signal indicative of the presence, or absence, of a train is referred to as a vital relay. A wide variety of circuits have been used to control vital relays and provide safe operating conditions.
One of the more obvious circuit considerations for actuating a vital relay is that it will be electrically energized when there is no train within the limits supervised by the track circuit. If the system depended upon the actuation of the relay in response to the presence of a train, there is the possibility that the relay would fail to operate for any number of reasons including: a power failure, a broken wire, dust or shorted equipment. Also, it is fairly obvious that the vital relay should be physically oriented so that gravity will move its contacts to the release position if it is not electrically actuated. Many other factors which are well known to those skilled in the art are considered in the design of track circuits and vital relays.
It is standard practice to apply a signal to the track near one end of the supervised boundary and to actuate a vital relay in response to the detection of the signal near the other end of the supervised boundary. The presence of a train within the supervised boundary is indicated by a shunt caused by the wheels and axle of the train between the two tracks.